The invention provides a method for preventing a flexible-package coiled lithium ion battery from deforming. The method does not comprises performing baking pretreatment on roll core assemblies (such as a cathode plate, an anode plate and a separator), and also does not comprise performing subsequent processing on roll cores. Due to the fact that the method does not comprise performing baking pretreatment nor subsequent processing on the roll core assemblies (such as a cathode plate, an anode plate and a separator), energy consumption is reduced, and also new adverse phenomena are avoided. The method fully considers factors leading to battery deformation, such as separator shrinkage, and cathode and anode plate thickness expansion caused by manufacture operations of baking, liquid injection, charging and the like, effective prevention on battery deformation is performed from the fountainhead (that is, the reeling operation), and S-shaped deformation is avoided. The method is simple in operation steps, convenient to operate and low in cost.